Centrosomes, which are composed of a pair of centrioles surrounded by an amorphous protein network of pericentriolar material (PCM), are fundamental cellular components that are critical for many cellular functions including microtubule assembly and cilia formation. Defective centrosome biogenesis causes male infertility, numerous types of cancer, and various developmental disorders. Examples of these developmental disorders are cystic kidney disease, Bardet-Biedl syndrome, left-right asymmetry, and Microcephaly. Clearly, proper centrosomes biogenesis is essential to human health. Not surprisingly, centrosome biogenesis is a complex, multi-step process. In particular, pericentriolar material (PCM) must assemble around a centriole for the resulting centrosome to function. The mechanisms regulating how PCM initially forms, is recruited to a centriole, and assembles around the centriole, remain a mystery; the proposal's goal is to solve that mystery. Achieving this goal will begin by focusing on a recently discovered cytoplasmic PCM complex. This complex includes multiple proteins known to be defective in patients suffering from Microcephaly, a disorder where brain size is severely reduced. The aims proposed here are to analyze (1) a potentially key interaction between Sas-4 and tubulin, (2), the role of Sas-4 complexes in cilia formation and (3) the role of Sas-4 complexes in astral microtubule formation. These aims will be accomplished using an interdisciplinary approach that combines genetics and biochemistry in the model organism Drosophila melanogaster. Drosophila possesses several favorable characteristics, which makes it ideal for studying centrosome biogenesis. First, numerous centrosome biogenesis mutants are currently available. Although many of these mutants are adult lethal, they are not embryonic or pupal lethal; thus, Drosophila is one of the only animals in which centrosome biogenesis mutants can be studied in detail. Second, Drosophila embryos can be collected in mass, which provides a plentiful source of the proteins involved in centrosome biogenesis; these proteins can then be used in biochemical and in vitro experiments. Third, other well-known characteristics of Drosophila, e.g., short generation time and ease of genetics, which have made it a preferred model organism for many biological systems, are also relevant to this project. These favorable characteristics of Drosophila should permit discovery of the mechanism of PCM formation and assembly; this discovery should aid in the development of diagnostic and treatment methods for human disorders caused by defective centrosome biogenesis.